Lefty or Righty? Genes for Handedness Found

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Genes that play a role in the orientation of internal organs may
also affect whether someone is right- or left-handed, new
research suggests.

The study, published today (Sept. 12) in the journal
PLOS Genetics, suggest those genes may also play a role in
the brain, thereby affecting people's handedness.

Still, the findings can't yet explain the mystery of why a
minority of people are left-handed because each gene only plays a
tiny role in people's handedness.

"Handedness is a complex trait, there are hundreds of genes
involved," said study co-author William Brandler, a genetics
doctoral candidate at the University of Oxford in England. "There
are also lots of environmental influences." [ Southpaw
Stats: 11 Fun Facts About Lefties ]

Origins of handedness

Throughout the world, between about 5 and 20 percent of the
population is left-handed. Favoring one hand for most tasks can
allow people to do things more quickly, but exactly why there's
such a strong bias towards right-handedness in humans is a
mystery. Humans' close relatives, such as chimpanzees, are
equally likely to be southpaws.

A 2012 study suggested that more cooperative societies that share
tools and tasks have more people with the
same dominant hand. Other studies propose that being a leftie
is handy in a fistfight — but only if most people are expecting a
right hook.

Although a few genes have been implicated in handedness, genes
are not the entire story, as
identical twins often favor different hands. Some have even
proposed that brain damage in utero causes brains to rewire to
make people lefties.

Genetic links

To get at the genetic roots of handedness, Brandler (who is a
southpaw) and his colleagues asked 728 people to move a row of 10
pegs using first their right hand, then their left. People who
take much longer on one side versus the other have greater hand
dominance.

The researchers then analyzed the genes of these people and
identified several genes associated with greater hand dominance.
They then confirmed the association in a larger group of 2,666
people.

The strongest association was with a gene called PCSK6 that
creates left- and right- parts in utero. The other genes played a
role in how the organs in the
body are oriented.

People with defects these genes may be otherwise healthy, but
have situs inversus, a condition in which internal organs are
mirrored from their normal orientation. Others have more serious
defects, such as left-handed isomerism, in which people have
essentially two left sides and multiple spleens throughout the
body, or heterotaxia, a typically fatal condition where "organs
are all over the place," Brandler told LiveScience.

The findings suggest that the same genes that affect the
left-hand symmetry of organs in the body also affects the way the
brain
is wired. That, in turn, affects whether someone's right or
left hand is dominant.